It is common in the oil and gas industry to collect data about the properties of the geologic layers within oil and gas wells by lowering sensing instruments down the well and taking measurements. Using various sensors, many different attributes of the subsurface may be measured and recorded, including electrical resistivity, conductivity, natural radioactivity, density, gravity, and temperature. Acoustic properties may be measured, and various types of seismic data collected, including VSP (Vertical Seismic Profile) data and microseismic data. It is also common when collecting seismic or microseismic data to place one or more geophones in boreholes drilled specifically for the purpose.
Some downhole sensors require that the sensor or some portion thereof be in physical contact with the sidewall of a borehole to increase coupling between the sensor and the borehole. In many cases, coupling may be achieved by pressing or clamping the sensor into firm contact with the sidewall of the borehole. Downhole sensors typically include transducers capable of sensing seismic signals such as geophones and accelerometers, which typically require good acoustic coupling to the borehole so that they may reliably and accurately receive and measure sound waves passing through adjoining geologic formations. Good acoustic coupling can sometimes be achieved by pressing the geophone into the sidewall of the borehole. This approach, however, introduces a directional variation in acoustic coupling, and therefore may not work well.
Another approach is to position an acoustic sensor centrally within a borehole while maintaining good acoustic coupling. In such an approach, for example, cement is poured into the borehole and fills the annular space between the sensor and the sidewall so that the sensor is firmly cemented in place. This provides efficient acoustic coupling between the sensor and the subsurface, and also maintains the sensor in a fixed orientation, which may be important for certain sensors.
Pouring cement into a borehole until the sensor is covered works well when a single sensor is located at the bottom of a borehole. This approach may not work so well when sensors are spaced at multiple points in a borehole, or are relatively close to the surface within a deep borehole. Conventional techniques can involve cementing sensors at such locations to achieve the desired acoustic coupling by filling almost the entire borehole with cement. There are some drawbacks to this approach, however. It is expensive, and requires more cement than would be needed just to ensure that the sensors are coupled to the sidewalls of the borehole. Moreover, for seismic or microseismic data being recorded with downhole geophones, there is a problem with noise being transmitted down the cement plug and the adjoining taut mechanical and electrical connections between adjoining downhole sensors, which together act as very efficient transducers, and pick up surface noise from well operations, roads, machinery, and the like, and broadcasting such noise deep into the subsurface. Such noise can also be reflected back to the geophones, and may overwhelm signals from desired seismic sources, especially in microseismic recording applications where the sources are relatively weak signals originating from naturally occurring faults and fracturing operations.
What is required are systems and methods of coupling downhole sensors to boreholes that overcome the problems associated with noise generation and propagation described above.